Environmental concerns have led to continued efforts to reduce NOx emissions of compression ignited (diesel) internal combustion engines. The latest technology being used to reduce the NOx emissions of heavy duty diesel engines is known as exhaust gas recirculation or EGR. EGR reduces NOx emissions by introducing non-combustible components (exhaust gas) into the incoming air-fuel charge introduced into the engine combustion chamber. This reduces peak flame temperature and NOx generation. In addition to the simple dilution effect of the EGR, an even greater reduction in NOx emission is achieved by cooling the exhaust gas before it is returned to the engine. The cooler intake charge allows better filling of the cylinder, and thus, improved power generation. In addition, because the EGR components have higher specific heat values than the incoming air and fuel mixture, the EGR gas further cools the combustion mixture leading to greater power generation and better fuel economy at a fixed NOx generation level.
Diesel fuel conventionally contains 300 to 400 ppm of sulfur, or more. Even the most recently contemplated “low-sulfur” diesel fuel will contain up to 50 ppm of sulfur (e.g. 10 to 50 ppm). When the fuel is burned in the engine, this sulfur is converted to SOx. In addition, one of the major by-products of the combustion of a hydrocarbon fuel is water vapor. Therefore, the exhaust stream contains some level of NOx, SOx and water vapor. In the past, the presence of these substances has not been problematic because the exhaust gases remained extremely hot, and these components were exhausted in a dis-associated, gaseous state. However, when the engine is equipped with an EGR system, particularly an EGR system in which the EGR stream is cooled before it is returned to the engine, the NOx, SOx, water vapor mixture is cooled below the dew point, causing the water vapor to condense. This water reacts with the NOx and SOx components to form a mist of nitric and sulfuric acids in the EGR stream.
In the presence of these acids, it has been found that soot levels in lubricating oil compositions build rapidly, and that under said conditions, the kinematic viscosity (kv) of lubricating oil compositions increase to unacceptable levels even in the presence of relatively small levels of soot (e.g. 3 wt. % soot). Because increased lubricant viscosity adversely affects performance, and can even cause engine failure, the use of an EGR system, particularly an EGR system that operates in a condensing mode during at least a portion of the operating time, requires frequent lubricant replacement. API-CI-4 oils developed specifically for EGR equipped HDD engines that operate in a condensing mode have been found to be unable to address this problem. It has also been found that simply adding additional dispersant is ineffective.
Therefore, it would be advantageous to identify lubricating oil compositions that better perform in passenger car and heavy duty diesel engines equipped with EGR systems, particularly EGR systems that operate in a condensing mode.
U.S. Pat. No. 6,715,473 to Ritchie et al. describes lubricating oil compositions for engines equipped with condensing EGR systems that contain certain polymeric materials found to control soot induced viscosity increase.
U.S. Pat. No. 6,869,919 to Ritchie et al. specifies lubricating oil compositions containing certain combinations of dispersants and detergents, and combinations of detergent and polymeric material that ameliorates soot induced viscosity increase.
While the above-noted patents describe means for reducing soot induced viscosity increase in lubricating oil compositions, particularly lubricating oil compositions that, with use, can be expected to become highly soot-loaded, additional solutions to the problem have been sought.
It is known that certain phenylenediamine compounds stabilize organic materials, including lubricating oils, against oxidative and thermal degradation.
U.S. Pat. No. 5,207,939 to Farng et al. describes certain reaction Mannich base reaction products of phenylenediamine, an aldehyde or ketone and a hindered phenol, which can be used in an antioxidant amount in lubricating oils, greases and fuel compositions.
U.S. Pat. No. 5,213,699 to Babiarz et al. describes certain N-allyl substituted p-phenylenediamine compounds useful as antioxidants for organic materials including lubricating oil compositions.
U.S. Pat. No. 5,298,662 to Smith et al. describes certain N-phenyl-p-phenylenediamines useful as antioxidants for polyol heat transfer fluids.
U.S. Pat. No. 5,232,614 to Colclough et al. describes substituted para-phenylenediamines as effective antioxidants for lubricating oil compositions.
While phenylenediamines were known to act effectively as antioxidants, these compounds were found to be disadvantageous commercially since the presence of such compounds, when used in amounts conventionally used to provide antioxidancy, displayed adverse effects on piston deposit and varnish control, and also displayed aggressiveness toward fluoroelastomeric engine seal materials. These adverse effects are particularly apparent with phenylenediamine compounds having higher nitrogen contents (compounds having relatively small hydrocarbyl substituents). Recent lubricating oil specifications for PCDO set by original equipment manufacturers (OEMs) have required reduced levels of lubricant phosphorus (e.g., <800 ppm). To date, lubricating oil specifications for heavy duty diesel (HDD) engines have not limited phosphorus content, although the next generation of lubricant specifications (e.g., API CJ-4) is expected to do so. Expected limits on phosphorus content (such as to 1200 ppm or less), and reductions in the allowable amounts of sulfated ash (SASH) and sulfur will limit the amount of zinc dialkyldithiophosphate (ZDDP), one of the most cost-effective antiwear/antioxidant compound, that a lubricant formulator can use. Reducing ZDDP levels requires formulators to employ increasing amounts of metal free (ashless) antioxidant, making the use of phenylenediamine as the primary antioxidant even less viable. Further, phenylenediamines are more costly than other available ashless antioxidants, specifically diphenylamines and hindered phenols.
Surprisingly, it has been found that with lubricating oil compositions containing at least one phenylenediamine compound, rapid soot-induced increases in lubricant viscosity associated with the use of engines provided with EGR systems can be ameliorated, even when such phenylenediamine compound is used in amounts at which the adverse affects of such compounds do not manifest.